Evolution in mammals, reptiles, birds and fish has developed extraordinarily efficient musculoskeletal systems for generating and controlling motion. However, the musculoskeletal system is not only an efficient system for delivering useful mechanical energy and load support, but is also capable of synthesizing, processing and organizing complex macromolecules to fashion tissues and organs for specific mechanical functions. An important subset of organs of the musculoskeletal system are the joints. Many types of joints exist in the body. Freely moving joints (ankle, elbow, hip, knee, shoulder, and those of the fingers, toes and wrist) are known as diarthrodial or synovial joints. The intervertebral joints of the spine are not diarthrodial joints as they are fibrous and do not move freely. They do, however, provide the flexibility required by the spine.
Diarthrodial joints enable local motion and other activities of daily life to take place. They perform their function so well that we are often not even aware of their existence nor the function they provide until injury strikes or arthritis develops. From an engineering point of view, these natural bearings are very uncommon structures. Under healthy conditions, they function nearly frictionless and remain almost entirely wear-resistant manner throughout our lives. Failure of the bearings surfaces of the body (i.e. articular cartilage), as with engineering bearings, means a failure of these bearings to provide their central functions. In biomedical terms, failure of diarthrodial joints leads to arthritis. The most common form being Osteoarthrisis or Osetoarthrothesis. Repair of arthritic joints means orthopedic surgery to replace the worn-out joints by a prosthesis or by a biological graft. This is an enormous medical and economic problem, with more than thirty million Americans suffering from the disabling disease.
Diarthrodial joints have some common structural features. First, all diarthrodial joints are enclosed in a strong fibrous capsule. Second, the inner surfaces of the joint capsule are lined with a metabolically active tissue, the synovium, which secretes the synovial fluid that lubricates the joint and provides the nutrients required by the avascular cartilage. Third, the articulating bone ends in the joint are lined with a thin layer of hydrated soft tissue known as articular cartilage. These linings, i.e. the synovium and articular cartilage layers, form the joint cavity which contains the synovial fluid. Thus, in animal joints, the synovial fluid, articular cartilage, and the supporting bone form the bearing system which provides the smooth nearly-frictionless bearing system of the body. While diarthrodial joints are subjected to an enormous range of loading conditions under cyclical conditions, the cartilage surfaces undergo little wear and tear under normal circumstances. Indeed, most human joints must be capable of functioning effectively under very high loads and stresses and at very low operating speeds. These performance characteristics demand efficient lubrication processes to minimize friction and wear of cartilage in the joint. Sever breakdown of the joint cartilage by either biochemical or biomechanical means leads to arthritis and thus arthritis is defined as a failure of the animal bearing system. Finally, the joint is stabilized by, and in motion is controlled by, ligaments and tendons which may be inside or outside the joint capsule.
Hyaline cartilage, as the names implies is glass smooth, glistening and bluish white in appearance, although older or diseased tissue tends to lose this appearance. The most common hyaline cartilage, and the most studied, is the articular cartilage. This tissue covers the articulating surfaces of long boned and siesamoid bones within diarthrodial joints. Articular cartilage is characterized by a particular structural organization. It consists of specialized cells (chondrocytes) embedded in an intercellular material (typically referred to as "cartilage matrix") which is rich in proteoglycans, collagen fibrils, other proteins, and water. Cartilage tissue is neither innervated nor penetrated by the vascular or lymphatic systems. However, in the mature joint of adults, the underlying subchondral bone tissue--which forms a narrow, continuous plate between the bone tissue and the cartilage--is innervated and vascularized. Beneath this bone plate, the bone tissue forms trabeculae, containing the marrow. In immature joints, articular cartilage is underlined by only primary bone trabeculae. A portion of the meniscal tissue in joints (referred to as the "interarticular" cartilage) also consists of cartilage whose make-up is similar to articular cartilage. It is generally believed that because articular cartilage lacks a vasculature, damaged cartilage tissue does not received sufficient or proper stimuli to elicit a repair response.
The menisci of the knee, and other similar structures such as the disc of the temporomandibular joint and the labrum of the shoulder, are specialized fibrocartilagenous structures which perform functions which are vital for normal joint function. They are known to assist the articular cartilage in distributing loads across the joint, assist the ligaments and tendons in providing joint stability, play a major role in shock absorption, and may assist in lubrication of the joint. Damage to these structures can lead to a reduction in joint function and degeneration of the articular cartilage. Surgical removal can result in early onset of osteoarthritis. The menisci, disc and labrum are hydrated fibrocartilage structures composed primarily of collagen (type I) with smaller amounts of other collagens and proteoglycans (including aggrecan and the smaller, non-aggregating proteoglycans). They contain a sparse population of cells which, like the chondrocytes of cartilage, are responsible for the synthesis and maintenance of this extracellular matrix.
Skeletal ligaments are specialized connective tissues that connect bones. They serve a passive mechanical function in stabilizing joints and in guiding joint motion. Further, they may have a neurosensory role transporting dynamic information to muscles. Ligaments are composed primarily of type I collagen organized in parallel arrays, with small amounts of other collagens, proteoglycans, elastin and other proteins and glycoproteins. The cells are fibroblastic in the midsubstance, and appear more chondroid at and near the insertion sites. Tendons have a similar structure, with a relatively high concentration of collagen, organized primarily as fibers in parallel. Other components are proteoglycans, elastin and other proteins and glycoproteins. The cells are fibroblastic in nature. The cells of the tendon and ligament are metabolically active and are responsible for the synthesis and maintenance of this extracellular matrix.
Proteoglycans comprise the second largest portion of the organic material in articular cartilage. These macromolecules are composed of a protein core to which are attached a number of covalently bound GAG chains (chondroitin sulfate and keratan sulfate). There are many different types of proteoglycans present in a wide range of tissues throughout the body; presumably, they also have different functions in the various tissues. However, the most extensively studied proteoglycans have been those from articular cartilage because of its role in regulating skeletal growth, joint function and the development of arthritis.
The major proteoglycans present in articular cartilage are the large aggregating type (50-85%) and the large non-aggregating type (10-40%) with distinct small proteoglycans also present. The molecular weights of these proteoglycan monomers range from 1.times.10.sup.6 to 4.times.10.sup.6, and they contribute significantly to the mechanical and physicochemical properties of cartilage. These molecules are highly ordered structures with length scales ranging from 10.sup.-8 to 10.sup.-6 m. First, there is an extended protein core with several distinct regions: an N-terminal region with two globular domains (G1 and G2), a keratan sulfate-rich domain, a longer chondroitin sulfate-rich domain which may also contain some interspersed keratan sulfate and neutral oligosaccharide chains, and a C-terminal globular domain (G3) on the proteoglycan monomer. Aggregates are formed when many proteoglycan monomers bind to a long monofilament chain of hyaluronan via their G1 globular domain. Each proteoglycan-hyaluronan bond is stabilized by a separate globular link protein (mw, 41,000-48,000), and this stabilization is vital in providing additional strength to the molecular network formed by proteoglycans in concentrated solutions. The structure of proteoglycan in cartilage is not uniform. The size of the proteoglycan aggregate depends mainly on the size of the hyaluronate chain and may be as large as 350.times.10.sup.6, particularly in fetal cartilage. Differences in chain length and amounts of keratan sulfate and chondroitin sulfate, length of the protein core, and degree of aggregation all contribute to the compositional and structural heterogeneties of proteoglycans within the tissue.
The GAG chains of the proteoglycans provide important physicochemical properties to cartilage. First, chondroitin sulfate (CS: .about.mw, 20,000) is composed of repeating disaccharide units of glucuronic acid and N-acetylgalactosamine with a sulfate (SO.sub.4) group and a carboxyl (COOH) group per disaccharide. Evidence exists indicating that these CS chains are the main determinants of frictional resistance against inter-stitial fluid flow. Keratan sulfate (KS) consists of repeating disaccharide units of galactose and N-acetylglucosamine, again averaging approximately one SO.sub.4 group per disaccharide. Keratan sulfate content of proteoglycans increases with age from zero in fetal cartilage to a maximum in aging cartilage. Both proteoglycan content and size decrease with increasing severity of OA.
In articular cartilage, molecular interactions occur through collagen-collagen covalent cross-link interactions, and proteoglycan-proteoglycan and collagen-proteoglycan non-covalent (electrostatic and mechanical) interactions. The most well-known interactions are the collagen-collagen covalent cross-links. These cross links are important in providing a strong and stiff collagen network. Thus, in the extracellular matrix these two molecular networks (proteoglycan and collagen) must coexist to form a fiber-reinforced composite solid with the collagen network providing the tensile stiffness and strength, and the proteoglycan network providing the compressive stiffness (via the Donnan osmotic pressure, electrostatic repulsion amongst the fixed negative charges and bulk compressive stiffness). The physical interactions between collagen and proteoglycan can arise from two sources: electrostatic and mechanical. First, evidence exists indicating that the negative charge groups on the proteoglycans can interact with the positive charge groups along the collagen fibrils, and hyaluronates of the aggregate do interact with type II, IX and X collagen. Second, evidence of strong frictional interaction between the proteoglycans and the fine collagen network also exists. No covalent bonding exists between collagen fibrils and proteoglycans. In normal cartilaginous tissue, proteoglycans are slowly but continuously turned over, the degraded molecules are released from the cartilage and are replaced by newly synthesized components. It is the coordinate control of synthesis and degradation of the matrix components by the chondrocytes that maintain normal cartilage. In experimental models of joint disease, for example, there is evidence of charges in the rate of biosynthesis and turnover of proteoglycans, which may contribute to cartilage degeneration. This chondrocyte-mediated degeneration leads to the whole cascade of degenerative bone and connective tissue events that results in osteoarthritis, limb immobilization, and other effects requiring surgical intervention.
Over one million surgical procedures in the United States each year involve cartilage replacement. Current therapies include transplantation with allografts (removing healthy cartilage from a donor, and reimplanting into a joint of the recipient), implantation of artificial polymer or metal prostheses, and surgical removal of old or degenerative cartilage and the surgical treatment of underlying bone to stimulate new cartilage formation. This new cartilage is usually a fibrous cartilage significantly inferior to the hyaline cartilage it is replacing. Other surgical procedures of synovial joints involve the replacement of menisci, ligaments and tendons with biological grafts or artificial tissues. Torn or severed menisci, discs of the temporomandibular joint, labrum of the shoulder, tendons and ligaments often undergo surgical repair. Degenerative loss of articular cartilage, for example, at the acetabular/femoral head artoculation, results in heavy loading of the soft tissue, and can require radical surgery.
Surgical procedures account for only a fraction of the treatment of individuals who suffer from disabling diseases resulting from connective tissue damage and degeneration in synovial joint. Alternative treatment includes conservative treatment (e.g., rest and physical therapy), and treatment is largely directed at symptomatic relief through the use of analgesics and nonsteroidal anti-inflammatory drugs.
There are significant limitations with all present approaches. Artificial prostheses have a limited lifetime, and can fail prematurely. Recurrent replacements of prostheses is not an advisable treatment, and, therefore, the relatively young and active patient is often consigned to slow joint degeneration until the use of prosthetic implants becomes a reasonable clinical option. Prostheses rarely replicate the performance of the original tissue. A prosthesis cannot adapt in response to environmental stress as does a biological tissue, nor can it repair itself. Biological allograft material is in limited supply, appropriate size shape and tissue type are difficult to obtain, and has the risk of carrying infectious diseases. Use of autograft material compromises the site used for the source tissue (e.g., using patella tendon to replace anterior cruciate ligament), and can only offer this tissue once. Biological graft tissue rarely, if ever, is able to provide a tissue with the same functional properties as the original tissue.
There appear to be genetic predisposition's to degenerate diseases of the connective tissues that exacerbate the effects of sport and other early injury. Even during pregnancy, there can be generalized stress on cartilage resulting from tropic hormone levels meant for softening of the pubic symphysis, and these can be exacerbated by congenital predisposition and prior injury.
There is therefore a need to offer a process that can stimulate the biological repair of the connective tissue, or stop the degeneration or slow the progression of degeneration of cartilage, menisci, tendons and ligaments.